Using counter-bore and capillary geometry to control mesophase pitch-based carbon fiber filament micro and macro structure

ABSTRACT

A spinning die for spinning mesophase pitch into carbon fibers. The die includes a central cavity having a bottom and at least one side wall which intersects the bottom. At least one capillary for forming a fiber is in fluid communication with the cavity, the capillary having a first open end, a second open end, an axis and a diameter (d). A flow disruption shoulder is positioned at the juncture of the bottom of the central cavity and the first open end of the capillary to create turbulence at the capillary entrance.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is directed to a process and apparatus for spinning fibers from mesophase pitches. In particular, the present invention is directed to counter-bore and capillary geometry which will create turbulence at the capillary entrance. The fibers generated according to the present invention have a non-radial cross-section and are predominantly free of longitudinal and helical cracking.

[0003] 2. Prior Art

[0004] The general methods and devices for melt and blow spinning fibers are well-known. Typically, in melt spinning, a spinnable substance is heated to a temperature which will allow it to flow. This substance then passes, usually under pressure, into a “spinning pack.” The spinning pack usually comprises a distribution plate and one or more spinning dies. A typical die will have a central cavity for receiving the spinnable substance and one or more capillaries or needles. The substance is distributed by the distribution plate to the central cavity of the die or dies, through the central cavity into the spinning capillaries and exits as fibers. Immediately upon exiting the capillary, the pitch fiber is attenuated usually by exerting tension on the fiber using a winding apparatus.

[0005] Blow spinning differs from melt spinning in that upon exiting the capillary, the fiber is contacted with an attenuating media, usually a gas. The attenuating media draws or stretches the fiber increasing its length while decreasing its diameter. Several types of dies are utilized for blow spinning fibers. Two common dies are annular and slot dies. Annular and slot dies differ primarily in the manner in which the attenuating gas is directed upon the exiting fiber.

[0006] The entrance wall from the central cavity into the capillary is typically a tapering entrance. The entrance wall may have a single angle between the wider central cavity and the capillary entrance or the entrance wall may have a compound angle with a first portion of the wall from the wider central cavity towards the capillary entrance having one angle and the second portion of the wall to the entrance of the capillary having a different angle.

[0007] Spinning of high quality fibers from mesophase pitches presents a number of unique problems. Mesophase pitches are comprised of molecules having aromatic structures which through interaction are associated together to form optically-ordered liquid crystals. Even when disturbed, the molecules have a visco-elastic memory which causes them to return to an ordered state.

[0008] The final arrangement of the molecules or graphitic plates across the cross-section of the carbon fiber or “texture” is developed during the spinning process and can take on a variety of patterns. The pattern of arrangement has a direct influence on fiber properties because the levels of residual stress within fibers having different patterns are markedly different.

[0009] The “radial” pattern is characterized by the basal plane radiating out from the center of the fiber like the spokes of a wheel, as shown in FIG. 2. The “onion-skin” pattern has basal plane “wrapping around” the center of the fiber, like a scroll. The “random” pattern is characterized by basal plane buckling and meandering across the fiber diameter in a random fashion. The fibrils of “radial” and “onion-skin” patterns tend towards parallel alignment with the axis of the fiber. In addition, there are “hybrid” fibers which exhibit a radial core with increasingly disoriented regions near the outer surfaces of the fiber.

[0010] During the spinning of fibers from mesophase pitches, the visco-elastic memory of the pitch tends to produce fibers having a radial cross-sectional structure. These fibers frequently develop longitudinal cracks rendering them undesirable for many applications. In general, these fibers have increased thermal and electrical conductivity and reduced tensile strength, and generally poor overall mechanical quality.

[0011] In applications requiring high strength, lower thermal conductivity and good stiffening characteristics, the preferred carbon fibers will have a non-radial cross-sectional structure. Production of these fibers requires maintaining mesophase pitch in a randomized state during the spinning process. Thus, to produce the desired fiber from a mesophase pitch, one must overcome the pitch molecules' visco-elastic memory or natural tendency to quickly return to an ordered state and maintain the molecules in a disturbed state until the pitch solidifies.

[0012] Small diameter carbon fibers are desirable because they follow the general relationship of increasing strength when measured as a function of cross-sectional area with decreasing diameter. Small diameter as-spun fibers are also desirable because the post-spinning processes of carbonization or graphitization require heat treatment and small diameter fibers can be heat treated more efficiently than large fibers.

[0013] Mesophase pitches also have a fairly short “spin-draw zone” or time during which the pitch fiber can be attenuated before it solidifies. This short “spin-draw zone” influences the diameter of the capillaries required to spin small diameter fibers and increases the difficulty of accurate machining of the spinning die. Small capillary diameters also decrease the length of the time period for continuous spinning before individual capillaries become “slow” or “plugged” and require cleaning and also make cleaning the spinning die more difficult.

[0014] As disclosed by Assignee's U.S. Pat. No. 5,259,947, solvated mesophase pitch provides significant advantages over traditional mesophase pitch, however, the unique characteristics of solvated pitch presents novel problems during the spinning of the fibers. Specifically, solvated mesophase pitch has rapid solidification times (and therefore a shorter spin-draw zone) in comparison to non-solvated pitches. Additionally, under spinning conditions of high throughput and low viscosity, solvated mesophase pitch has rapid molecular response times in comparison to non-solvated mesophase pitch. As a result of the rapid molecular response times, solvated pitch also has a short visco-elastic memory, i.e., if disrupted or randomized the pitch molecules or graphitic plates will quickly return to an ordered state.

[0015] A number of approaches have been used to disrupt the flow of the pitch molecules to overcome the visco-elastic memory of mesophase pitches so that the spinning of fibers having a non-radial cross-section is possible.

[0016] One approach is to disrupt the pitch flow before it enters the central cavity or within the central cavity using mechanical means. For example, U.S. Pat. Nos. 5,169,584 and 5,202,072 and Assignee's Patent Nos. 5,437,927 and 5,578,330 disclose a process for melt spinning small diameter high strength fibers from mesophase pitch by passing the pitch through a restriction plate located between the distribution plate and the central cavity.

[0017] U.S. Pat. Nos. 4,717,331, 4,775,589, 4,818,449 and 5,547,363 disclose methods for spinning non-radial cross-section fibers from mesophase pitch using melt spinning dies having various inserts within the central cavity. U.S. Pat. No. 4,717,331 utilizes a molded insert, U.S. U.S. Pat. No. 4,775,589 utilizes a mechanical stirrer, U.S. Pat. No. 4,818,449 utilizes a spiral insert such as a drill bit or worm gear and U.S. Pat. No. 5,547,363 utilizes a spiral insert such as a metal spring.

[0018] A second approach to disrupting the pitch flow is to use a flow disruption medium within the central cavity between the central cavity and the capillary or within the capillary.

[0019] U.S. Pat. No. 4,816,202 discloses a process for spinning carbon fibers having a non-radial cross-section from mesophase pitch using a melt spinning die having a flow disruption means comprising a particulate layer within the central cavity.

[0020] U.S. Pat. No. 4,923,648 discloses a process for spinning a fiber having a uniform mesh non-radial cross-section from mesophase pitch utilizing a melt spinning die which has a fine mesh layer within the central cavity.

[0021] Assignee's U.S. Pat. No. 5,766,523 discloses a process for spinning carbon fibers having a non-radial cross-section from solvated mesophase pitch using a blow spinning die having a disruption means such as particles, fibers or screens within the capillary or between the central cavity and the capillary.

[0022] A third approach to disrupting the pitch flow by is by altering the shape or geometry of the capillary.

[0023] U.S. Pat. Nos. 4,814,121 and 4,913,889 disclose a melt spinning die to produce non-radial fibers from mesophase pitch, the die having a capillary outlet cross-sectional area greater than the cross-sectional area of the narrowest part of the capillary.

[0024] U.S. Pat. No. 4,859,381 discloses a process for melt spinning mesophase pitch using a die in which the pitch is passed through a first capillary portion having a specific shape designed to exert shearing stress on the pitch, then through a second capillary portion designed to temporarily hold the pitch in a state free of shearing stress prior to exiting the capillary.

[0025] U.S. Pat. No. 4,816,202 discloses a melt spinning die for spinning non-radial fibers from mesophase pitch which has, in addition to other features described below, a capillary length-to-diameter ratio (L/d) of from 0.5 to 10.

[0026] Assignee's U.S. Pat. No. 5,766,523 discloses a process for spinning fibers having a random cross-section using a blow spinning die having, in addition to a flow disruption means, a capillary length-to-diameter ratio (L/d) of between from about 2 to about 10.

[0027] A fourth approach is to alter the geometry of the central cavity and/or change the relationship between the central cavity and the capillary.

[0028] U.S. Pat. No. 4,576,811 discloses that the cross-sectional structure of melt spun mesophase pitch fibers change from radial to onion skin when the entrance angle from the central cavity to the capillary is in the range of 120 to 170 degrees (60 to 10 degrees from the capillary axis).

[0029] U.S. Pat. No. 4,816,202 discloses a melt spinning die for spinning non-radial fibers from mesophase pitch having an entrance angle from the central cavity to the capillary between 45 and 100 degrees relative to the axis of the capillary, a central cavity depth between 5 and 200 times the diameter of the capillary, and an average distance from the axis of the capillary to the nearest side wall of the central cavity of 1 to 25 times the diameter of the capillary.

[0030] U.S. Pat. No. 5,037,589 discloses a melt spinning die which produces non-radial or modified radial fibers from mesophase pitch by having only one capillary per central cavity and placing the capillary entrance in a position asymmetrical to the central cavity.

[0031] Use of a disruption medium decreases the continuous spinning time possible and increases maintenance time and cost since replacement of the disruption medium may be difficult and time-consuming. Use of a disruption medium may also result in an undesirable pressure drop across the medium. Furthermore, placement of the disruption medium adjacent to the entrance of the capillary may result in increase in the incidence of “slow” or plugged capillaries.

[0032] Altering the geometry of the capillary or changing the relationship of the capillary to the central cavity increases the difficulty of accurate machining and increases the difficulty of cleaning the die. Furthermore, use of a capillary having an outlet cross-sectional area greater than the cross-sectional area of the narrowest part of the capillary is ineffective for spinning solvated mesophase pitches because the low viscosity of the pitch and pressure drop at the capillary outlet result in buildup of pitch at the outlet.

[0033] In summary, there remains a need for a spinning die which will disrupt the flow of pitch as it passes through the capillary which has a design which can be machined accurately. There is a need for a spinning die which has a design which can be machined more economically. There remains a need for a spinning die which will disrupt the flow of pitch as it passes through the capillary having a geometry which will be combined with the passages for the attenuating gases.

[0034] Further, there is a need for a spinning die which will permit continuous spinning for longer periods of time. There further remains a need for a spinning die which is simple and less expensive to maintain.

[0035] For the purposes of this specification and claims, the following terms and definitions apply:

[0036] “Capillary” that portion of a blow spinning slot die which forms a spinnable substance such as a solvated pitch into a fiber. For the purposes of this disclosure, the term “capillary” also includes the term “needle” or “spinning needle” or “spinneret” as commonly used in blow spinning dies and other spinning die types.

[0037] “Carbon fibers” are fibers following carbonization and/or graphitization.

[0038] “Draw-down time” is the time span between the time point at which a liquid pitch in the form of a fiber exits a spinning die and the time point at which the pitch solidifies. Draw-down time defines the time and distance over which a pitch fiber can be attenuated or drawn to decrease the diameter of the fiber to less than the diameter of the fiber as it leaves the spinning die.

[0039] “Fibers” means lengths of fiber capable of formation into useful articles.

[0040] “Fluid Temperature” for a solvated pitch is determined to be the temperature at which a viscosity of 600 poise is registered upon cooling of the solvated pitch at 1 degree Celsius per minute from a temperature in excess of its melting point. If the melting point of a solvated pitch could be easily determined, it would be lower than the fluid temperature.

[0041] “Isotropic pitch” means pitch comprising molecules which are not aligned in optically ordered liquid crystal.

[0042] “Mesophase pitch” means pitch comprising molecules having aromatic structures through which interaction is associated to form optically ordered liquid crystals, which are either liquid or solid depending on temperature. Mesophase pitch is also known as anisotropic pitch.

[0043] “Petroleum pitch” means the residual carbonaceous material obtained from the catalytic and thermal cracking of petroleum distillates or residues.

[0044] “Pitch”, as used herein, means substances having the properties of pitches produced as by-products in various industrial production processes such as natural asphalt, petroleum pitches and heavy oil obtained as a by-product in a naphtha cracking industry and pitches of high carbon content obtained from coal.

[0045] “Pitch central cavity” or “central cavity” as used herein means the central cavity of a spinning die. The central cavity distributes the fluid pitch to one or more capillaries. “Pitch central cavity” as used herein is synonymous with “pitch flow section”.

[0046] “Pitch fibers” or “Pitch carbon fibers” are as spun fibers prior to carbonization or oxidation.

[0047] “Solvated pitch” or “Solvated mesophase pitch” means a pitch which contains between 5 and 40 percent by weight of solvent in the pitch. Solvated pitch has a fluid temperature lower than the melting point of the pitch component when not associated with solvent. Typically, the fluid temperature is lowered by about 40 degrees Celsius.

BRIEF SUMMARY OF THE INVENTION

[0048] The present invention provides a blow spinning die especially suited for spinning carbon fibers from solvated pitches. A cross-sectional view of fibers prepared with this die shows a non-radial orientation of the graphitic plates which comprise the fiber.

[0049] A typical blow spinning die has a central cavity for receiving a spinnable substance. However, the cavity may vary in geometry and in some cases may be eliminated. Additionally, the die will contain at least one capillary which receives the pitch and forms it into a fiber as it passes out of the die.

[0050] The present invention provides a blow spinning die especially suited for spinning fibers from a solvated pitch. This die provides a novel geometry of the central cavity at the juncture of the central cavity and the entrance of the capillary which creates turbulence in the pitch. The turbulence created disrupts the molecular orientation of the pitch and the resulting disorder of the graphitic plates yields a fiber having a non-radial cross-sectional structure.

[0051] Additionally, the present invention provides an improved process for blow spinning carbon fibers from solvated pitches. The improved process of the present invention produces fibers having a non-radial cross-sectional structure. According to the improved process of the present invention, a spinnable solvated pitch is heated to temperature sufficient to allow it to flow. The pitch passes into a blow spinning die and exits the fiber through a capillary as a fiber. Upon exiting the capillary, the fiber is attenuated. The improvement provided by the present invention comprises passing the solvated pitch through a blow spinning die which provides a novel geometry of the central cavity at the juncture of the central cavity and the entrance of the capillary.

[0052] The present invention further provides a pitch fiber which has its internal molecules or graphitic plates arranged in a randomized manner. Following carbonization, the fiber will have a non-radial cross-sectional structure when viewed under a scanning electron microscope. The carbon fibers provided by the present invention have improved tensile strength, improved strain to failure ratio, shear modulus, improved handling ability and lower thermal conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0053]FIG. 1 depicts a fiber of the present invention having a non-radial cross-section.

[0054]FIG. 2 depicts a fiber of the prior art having a radial cross-section.

[0055]FIG. 3 depicts a fiber of the prior art having a radial cross-section and showing a longitudinal crack.

[0056]FIG. 4 is a perspective view of a blow spinning die constructed according to the present invention.

[0057]FIG. 5A is a partial view of a cross-section of a blow spinning die according to the present invention from the perspective of section line 5-5 of FIG. 4.

[0058]FIG. 5B is a partial view of a cross-section of a blow spinning die in an alternate configuration.

[0059]FIG. 5C is a partial view of a cross-section of a blow spinning die in an alternate configuration.

[0060]FIG. 6 is a partial view of a longitudinal cross-section through the die tip of a blow spinning die according to the present invention from the perspective of section line 6-6 of FIG. 4.

[0061]FIG. 7 is a partial top view of a blow spinning die according to the present invention from the perspective of section line 7-7 of FIG. 4.

[0062]FIG. 8 is a top view of a die according to the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0063] The embodiments discussed herein are merely illustrative of specific manners in which to make and use the invention and are not to be interpreted as limiting the scope of the instant invention.

[0064] While the invention has been described with a certain degree of particularity, it is to be noted that many modifications may be made in the details of the invention's construction and the arrangement of its components without departing from the spirit and scope of this disclosure. It is understood that the invention is not limited to the embodiments set forth herein for purposes of exemplification. Like numbers in the drawings indicate like parts in various embodiments of the invention.

[0065]FIG. 1 illustrates a fiber produced according to the present invention having a non-radial cross-section while FIGS. 2 and 3 illustrate fibers of the prior art.

[0066] Referring to FIG. 4, the preferred embodiment of the present invention is a blow spinning die 20 which has a central cavity 22 which acts as a pitch reservoir and a die tip 24. The die tip will have a plurality of capillaries fed by the central cavity. In use, the dies will be removable for cleaning and replacement. In the preferred embodiment, spinning die 20 is a slot die, however as can be appreciated by one skilled in the art, the present invention is equally applicable to annular dies, or arcuate dies.

[0067] Referring to FIG. 5A, spinning die 20 has a central cavity 22 which is in fluid communication with a capillary 26. Capillary 26 has a first open end 28, a second open end 30 and an axis, represented by dashed line 32. Capillary 26 is cylindrical with a radius r and has a capillary wall 34 which is parallel to axis 32 throughout its length. The length of capillary 26 is represented by symbolic arrow 36. In the preferred embodiment capillary 26 has a ratio of length to radius (L:d) of from about 2:1 to about 10:1, and more preferably from about 3:1 to about 4:1.

[0068] Central cavity 22 has a side wall 38, and has a bottom 40 which is essentially flat. The angle between bottom 40 and capillary axis 32 is represented by symbolic arrow 42, and in the preferred embodiment for solvated mesophase pitch is 90 degrees.

[0069] The point at which side wall 38 intersects bottom 40 is indicated by symbolic arrow 44 measured as the distance between axis 32 and intersection 44 or the distance to the nearest sidewall, is represented by dotted line 46 and symbolic arrow 48.

[0070] The angle between axis 32 and side wall 38 is represented by symbolic arrow 50. Side wall 38 may have a single angle 50 from the top of central cavity 22 and intersection 44 with bottom 40 or a compound angle with the upper portion of the side wall having a different angle 50 than the lower portion of the side wall 38 which intersects with bottom 40. The central cavity 22 of the preferred embodiment has a side wall 38 which comprises a compound angle 50, with the upper portion of central cavity 22 having side walls 38 which are essentially parallel to axis 32 and the lower portion of central cavity 22 having side walls 38 in which angle 50 is preferably between about 15 degrees and about 25 degrees.

[0071] The juncture between central cavity 22 and the first open end 28 of capillary 26 forms a disruption shoulder and is represented by arrow 52. The angle at which capillary wall 34 intersects bottom 40 is represented by arrow 54. In the preferred embodiment angle 54 is 90 degrees and juncture 52 is a sharp edge. As can be appreciated by one skilled in the art, rounding or chamfering of the edge at juncture 52 will decrease the desired disruption of the pitch as it flows from central cavity 22 into capillary 26. As can also be appreciated by one skilled in the art, decreasing the angle at which capillary wall 34 intersects bottom 40 to less than 90 degrees may result in increased turbulence at juncture 52.

[0072] In the blow spinning die of the preferred embodiment, attenuating gas flows along the outside surfaces 56 of die tip 24 as shown by arrows 57 to contact and attenuate the fiber as it leaves the second open end 30 of capillary 26. The angle between capillary axis 32 and the outside surface 56 of die tip 24 is indicated by arrow 58. In the preferred embodiment angle 58 is less than 45° (45 degrees). The angle 58 will be the angle at which the attenuating gas contacts the fiber. As can be appreciated by one skilled in the art, angle 58 is critical in the application of the present invention to a blow spinning die.

[0073] The geometry of the die should maximize the number of capillaries in the minimum size of the die while providing an arrangement to deliver attenuating gas at a proper angle.

[0074]FIGS. 5B and 5C depict alternate embodiments of the present invention in which sidewall 38 comprises a single angle 50 from the top of central cavity 22 to intersection 44 from approximately 0° to 30°.

[0075] Referring to FIGS. 6 and 7, the relationship between two adjacent capillaries in a slot die is illustrated. In FIG. 7, the axis of capillary 26 is represented by arrow 32. The distance between the axes of two adjacent capillaries is represented by arrow 60. By way of example, the distance between the axes of adjacent capillaries required to implement the present invention is at least one diameter plus one mil. As will be appreciated by one skilled in the art, the distance between capillaries may be increased according to the type of spinning die and the configuration of the post-spinning apparatus, as well as economics. In the preferred embodiment of a blow spinning die with a slot central cavity, the distance between the axes of adjacent capillaries is approximately 30 mils.

[0076]FIG. 8 depicts a top view of a typical single capillary or an annular die according to the present invention. In this configuration, there is one capillary for each central cavity and there may be multiple central cavity/capillary pairs in the spinning die. Die 80 in this figure is depicted as having a central cavity which is frusto-conical, however one skilled in the art will appreciate that the central cavity may have other shapes. As described in the previous figures, bottom 40 is essentially flat and essentially perpendicular to axis 32. The distance to the nearest side wall, measured as the distance between capillary axis 32 and juncture 44 between bottom 40 and side wall 38, is indicated by symbolic arrow 48 and is the capillary radius plus at least one mil.

[0077] Using a blow spinning die constructed in accordance with the present invention, solvated mesophase pitch heated to a temperature wherein the viscosity of the pitch was 195 poise may be continuously spun into small diameter fibers (<17 g) having a non-radial cross-section.

[0078] Whereas, the present invention has been described in relation to the drawings attached hereto, it should be understood that other and further modifications, apart from those shown or suggested herein, may be made within the spirit and scope of this invention. 

What is claimed is:
 1. A spinning die for spinning mesophase pitch into carbon fibers comprising: a central cavity having a bottom and at least one side wall which intersects said bottom; at least one capillary for forming a fiber in fluid communication with said cavity, said capillary having a first open end, a second open end, an axis and a diameter (d); and a flow disruption shoulder positioned at the juncture of said bottom of said central cavity and said first open end of said capillary.
 2. The spinning die of claim 1 wherein said flow disruption shoulder is essentially perpendicular to the axis of said capillary.
 3. The spinning die of claim 1 wherein the ratio of the length of said capillary to the diameter of said capillary (L:d) ranges from about 2:1 to about 10:1.
 4. The spinning die of claim 1 wherein the ratio of the length of said capillary to the diameter of said capillary (L:d) ranges from about 3:1 to about 4:1.
 5. The spinning die of claim 1 wherein the side wall and the bottom of said central cavity meet at an angle ranging from 0 degrees to less than 90 degrees relative to the axis of said capillary.
 6. The spinning die of claim 1 wherein the side wall and the bottom of said central cavity meet at an angle ranging from about 10 degrees to about 30 degrees relative to the axis of said capillary.
 7. The spinning die of claim 1 wherein the distance between the axis of said capillary and the intersection of the nearest side wall with the bottom of said central cavity is at least one radius plus one mil.
 8. The spinning die of claim 1 wherein there is more than one said capillary and the distance between the axes of adjacent said capillaries is at least one diameter plus one mil.
 9. A spinning die as set forth in claim 1 wherein said flow disruption shoulder comprises a sharp edge.
 10. The spinning die of claim 1 wherein said die is a slot or annular die.
 11. A blow spinning die for spinning mesophase pitch into carbon fibers comprising: a die body having at least one central cavity, said cavity having a bottom and at least one side wall which intersects said bottom; a die tip, said die tip containing at least one capillary for forming a fiber, said capillary being in fluid communication with said central cavity, said capillary having a first open end, a second open end, an axis and a diameter (d), said capillary exiting said die body at the terminus of said die tip; a flow disruption shoulder positioned at the juncture of said bottom of said central cavity and said first open end of said capillary; and means for directing at an attenuating gas stream at an acute angle to said diameter (d) to the region adjacent to said terminus of said die tip.
 12. The blow spinning die of claim 11 wherein said flow disruption shoulder is perpendicular to the axis of said capillary.
 13. The blow spinning die of claim 11 wherein said acute angle is less than 45°.
 14. The blow spinning die of claim 11 wherein the ratio of the length of said capillary to the diameter (L:d) ranges from about 2:1 to about 10:1.
 15. The blow spinning die of claim 11 wherein the ratio of the length of said capillary to the diameter (L:d) ranges from about 3:1 to about 4:1.
 16. The blow spinning die of claim 11 wherein the angle of intersection between the side wall and the bottom of said central cavity ranges from 0 degrees to less than 90 degrees relative to the axis of said capillary.
 17. The blow spinning die of claim 11 wherein the angle of intersection between the side wall and the bottom of said central cavity ranges from about 10 degrees to about 30 degrees relative to the axis of said capillary.
 18. The blow spinning die of claim 11 wherein the distance between the axis of said capillary and the intersection of the nearest side wall with the bottom of said central cavity is at least one radius plus one mil.
 19. The blow spinning die of claim 11 wherein there is more than one said capillary and the distance between the axes of adjacent capillaries is at least one diameter plus one mil.
 20. The blow spinning die of claim 11 wherein said flow disruption shoulder comprises a sharp edge.
 21. The blow spinning die of claim 11 wherein said die is a slot or annular die.
 22. A process for spinning carbon fibers from mesophase pitch, said fibers having a round cross-section and a non-radial structure comprising: heating a spinnable pitch to a temperature sufficient to allow the pitch to flow; and passing the pitch into a spinning die, said die having a central cavity and at least one capillary in fluid communication therewith; and drawing said pitch past a flow disruption shoulder positioned at the juncture of said central cavity and said capillary; and drawing said pitch from said capillary by force of an attenuating gas to form a fiber.
 23. A process for spinning carbon fibers as set forth in claim 22 wherein said mesophase pitch is a solvated mesophase pitch.
 24. A process for spinning carbon fibers as set forth in claim 22 including the step of contacting said attenuating gas with said pitch at an angle less than 45°.
 25. A process for spinning carbon fibers as set forth in claim 22 including disrupting said pitch by said shoulder through the complete length of said capillary.
 26. A process for spinning carbon fibers as set forth in claim 22 wherein said flow disruption shoulder is perpendicular to the axis of said capillary. 